1. Field of the Invention
The present invention relates to a method for producing an enriched gas having the concentration of a certain gas increased comprising separating the certain gas from a feed gas mixture by applying a pressure difference to an adsorbent selective for the certain gas, and an apparatus therefor.
2. Description of the Related Art
The method for concentrating a certain gas using an adsorbent to increase the concentration of the gas is representatively classified into a method using the difference between transit rates of gases by applying a pressure to a gas separation membrane and a method of PSA (Pressure Swing Adsorption) type using a zeolite molecular sieve (ZMS) or a carbon molecular sieve (CMS), in which a pressure difference is applied to a container packed with the sieve material, whereby a gas of a certain component is adsorbed to the adsorbent while a gas of a relatively less adsorbable component is separated. The PSA has been widely used in various fields since the proposal by Skarstrom.
According to the PSA type, the adsorption of a gas is carried out at normal(atmospheric) pressure or higher and the desorption and regeneration is carried out at atmospheric pressure. The PSA which has been developed since 1950's is widely used in the industrial production of oxygen and nitrogen and recently is also applied to a small size articles such as a water purifier, an air conditioner, an air cleaner and a medical appliance, as well as an air drying, an oxygen concentration and a hydrogen purification. Oxygen obtained from the PSA is widely used in the industries which continuously consumes oxygen, concretely the electric arc steel making process, the wastewater treatment by adding air to the wastewater, the pulp bleaching, an ozone generator and the like. In recent, an oxygen-enriched gas instead of air is used in combustion for the purpose of low nitrification (NOx) and high efficiency and the biochemical field such as fermentation. Particularly, the oxygen concentrator has been applied in electric home appliances for air conditioning in an office and at home. FIG. 1 shows a representative Skarstrom cycle of a standard basic four-step process of the PSA using two adsorbent beds. Here, Pl and Ph represent a relatively low pressure value and a relatively high pressure value, respectively, of the operating pressure. The process comprises essentially a feed pressurization step, a blowdown step and a purge step. The adsorption is carried out during the feed pressurization while the desorption is carried out during the blowdown step and the purge step. The typical O2-PSA processes are described in, for example, U.S. Pat. No. 3,430,418; U.S. Pat. No. 4,589,888; U.S. Pat. Nos. 4,650,501 and 4,981,499. On the basis of the standard process, a pressure equalization step may be further added to construct a multi-step process. Since such multi-step process may show a severe fluctuation in discharge pressure, the number of the adsorbent beds should be increased to reduce the deviation of the fluctuation. The multi-bed system, though has a high efficiency, has a limitation in the size reduction. The PSA apparatus is advantageous for high concentration systems for medical use but disadvantageous for the electric home appliance aiming at the output rather than the concentration. Also, it has a defect in that the initial production cost is relatively high.
For example, referring to the conventional method (JP-A-No. 08-239204), as shown in FIG. 7, there are required a plurality of valves (14, 12a, 17a, 12b, 17b) or line (15) in an input port and also a plurality of valves (110a, 110b) in a production port. Also, a pressure equalizing line 112 for connecting an adsorbent bed A and adsorbent B is provided with a switch valve 113. Further, the pressure equalizing line 112 is provided with a detour pressure equalizing line 115 equipped with a locking means 114, which makes the structure complicated. An enriched gas is produced from lines 19a and 19b. 
The PSA is subdivided into an original PSA type operated at atmospheric pressure or higher, a VPSA type operated at between atmospheric pressure or higher and vacuum pressure and a VSA type operated at atmospheric pressure or low, according to the operating pressure. The VSA type and the VPSA are exchangeably used, or inclusively designated the PSA. Examples of the V(P)SA type are disclosed in U.S. Pat. No. 5,122,164; U.S. Pat. No. 5,223,004 and U.S. Pat. No. 5,246,676.
Representative evaluation factors of the PSA are concentration, recovery rate and productivity. In the conventional methods, the process design and system design are laid out considering mainly concentration, recovery rate and the initial investment in accommodation and operating cost, since they are focused on their industrial application. The RPSA (Rapid PSA) which has been developed in 1970's focusing on the output of a product gas could produce an oxygen-enriched gas in a several times great amount using pressure depression generated in an adsorbent bed packed with fine particles, unlike most of the PSA, in which the depression inside the adsorbent bed is absent or ignored. That is, since a desired gas is continuously produced during the depression step unlike the common PSA, it is possible to increase the recovery rate of the desired gas and the productivity of the adsorbent. FIG. 2 shows the standard three-step process of the RPSA comprising a pressurization step, a delay step and a depressurization step. The delay step as a middle step may be omitted, where appropriate. As shown in FIG. 2, the production is continued in the respective steps and the pressurization step and the depressurization step take place in a short time of several seconds. Typical examples of the RPSA are disclosed in U.S. Pat. No. 4,406,675; U.S. Pat. No. 5,827,358; U.S. Pat. No. 6,068,680 and U.S. Pat. No. 6,565,627. The method of this type has been partially applied in an emergency oxygen supply apparatus for a pilot and some industrial uses but not yet applied in commercial small size home appliances.
For a small capacity oxygen concentrator to be combined with an air cleaner and an air conditioner for domestic use, the high concentration of the industrial use is actually meaningless since the ultimate oxygen concentration of a target space is at a level of about 21 to 23% where people feel pleasantness. For a water purifier, it is known that an oxygen concentration of 50% or higher is sufficient to increase the dissolved oxygen. Therefore, the adoption of the RPSA method which can accomplish high productivity is not preferable for a small size machine.
The PSA operated at a pressure of at least normal pressure shows good efficiency when constructed in a small size. However, when a commonly used pressure of 2 atm to 5 atm is used, problems of noise and heat generation of a pump and durability may occur. Therefore, it cannot be used in an electric home appliance for indoor use. Recently, there have been proposed methods for increasing recovery rate while reducing the adsorption/desorption pressure at maximum, such as U.S. Pat. No. 5,074,892, U.S. Pat. No. 6,010,555 and U.S. Pat. No. 6,506,234. However, these methods have limitations since a high performance adsorbent should be used and the adsorbent are hardly recycled. Also, since the methods are not focused on small size machines and productivity, they are only suitable for medical use but not for application in a small size home appliance. In order to be applied to a small size home appliance, a process to make the maximum productivity at low power consumption is needed.
The VPSA operated at between atmospheric pressure and vacuum pressure can guarantee a more quiet operation than the PSA. However, as described above, the conventional method has difficulties in the construction at a low cost due to the complex multi-step control and the use of a valve apparatus and a surge tank.
In the absolute VSA only operated at atmospheric or lower, the adsorption and desorption is carried out by means of a main vacuum pump and a concentrated gas is supplied using an air blower. Thus, this method is advantageous in that the problems related with noise and heat generation of a vacuum pump may work out considerably since the pressure difference applied on an adsorbent is small. However, two pumps should be essentially needed in the construction of a machine, which makes this method economically disadvantageous. Also, upon application in a small size machine, it is necessary to use a high performance adsorbent, since the productivity is low. Further, when 3or more of adsorbent beds are used in the construction of a machine, the valve control becomes complicated and thus is not suitable for use in a small size air conditioner.